Making an anode assembly

ABSTRACT

An electroplating system for cathodically plating an epitrochoidally shaped internal surface of a rotary engine housing. An anode assembly is provided which is comprised of a perforate walled container of titanium metal or other anodically inert metal to which a voltage potential can be applied; the basket contains anode pieces such as nickel which are shaped to be in intimate contact with each other during the plating operation. The perforate walls of the anode container is shaped from flexible expanded titanium sheet metal interfitted within semi-epitrochoidally aligned grooves respectively machined into titanium plates forming the ends of the anode assembly. The anode walls are thus shaped substantially complimentary to the epitrochoid configuration of the cathode but have a predetermined deviation adjacent the nodes of the trochoid for insuring a uniform but heavy coating thickness under high speed electroplating conditions.

' United States Patent 1191 Cordone et al.

[ Dec. 16, 1975 1 MAKING AN ANODE ASSEMBLY Mich.

[73] Assignee: Ford Motor Company, Dearborn,

Mich.

[22] Filed: July 1, 1974 [21] Appl. No.: 484,727

Related US. Application Data [62] Division of Ser. No, 413,154, Nov. 5,1973, Pat. No.

[52] US. Cl. 204/283; 204/259; 204/260; 204/272; 204/284 [51] Int. Cl.C25D 17/10; C25D 7/04; C25D 3/12 Stephan et al. 204/237 Stephan et al.204/38 B Primary Examiner-F. C. Edmundson Attorney, Agent, orFirm-Joseph W. Malleck; Keith L. Zerschling [5 7] ABSTRACT Anelectroplating system for cathodically plating an epitrochoidally shapedinternal surface of a rotary engine housing. An anode assembly isprovided which is comprised of a perforate walled container of titaniummetal or other anodically inert metal to which a voltage potential canbe applied; the basket contains anode pieces such as nickel which areshaped to be in intimate contact with each other during the platingoperation. The perforate walls of the anode container is shaped fromflexible expanded titanium sheet metal interfitted withinsemi-epitrochoidally aligned grooves respectively machined into titaniumplates forming the ends of the anode assembly. The anode walls are thusshaped substantially complimentary to the epitrochoid configuration ofthe cathode but have a predetermined deviation adjacent the nodes of thetrochoid for insuring a uniform but heavy coating thickness under highspeed electroplating conditions.

4 Claims, 4 Drawing Figures few/r1! div 42 1172 Z4 [Err/429174 if ar /4xIf US. Patent Dec. 16, 1975 Sheet 1 of2 3,926,772

US. Patent Dec.16,1975 Sheet2of2 3,926,772

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MAKING AN ANODE ASSEMBLY This is a division of application Ser. No.413,154, filed Nov. 5, 1973, now U.S. Pat. No. 3,891,534.

BACKGROUND OF THE INVENTION It is generally well known in electroplatingthat the density of current flow will be uneven at the sharp edges orcontour changes on the object to be plated. This phenomenon involves aproposition that there is increased plating resulting from increasedcurrent density at any outstanding contour, while the opposite effectwill take place at depressions. In the latter case, the density ofcurrent fiow becomes less than the average density of plating currentover the full area being treated. This problem becomes exaggerated whenan article to be plated has a compound curvature, such as in anepitrochoid, where the cathode is able to receive current throw from twodifferent zones or anode locations due to the reverse or compoundcurvature. Accordingly, certain areas will be unduly thick because ofthe throwing power which is multiplied in some areas.

In applications such as a functional coating for a wear surface of aninternal combustion engine, i.e. the internal rotor housing surface of arotary internal combustion engine, the need for uniformity in thecoating is extremely severe. The efficient electroplater not only seeksto obtain uniform thickness in such applications, but the plating mustbe of good sound density throughout; the latter will be degraded as aresult of inappropriate bath chemistry, electrode spacing, and change ofthe anode or cathode area during the plating process.

SUMMARY OF THE INVENTION A primary object of this invention is toprovide an anode assembly useful in an electroplating system forcathodically plating an article having a compound or reverse curvature,the anode assembly being particularly adapted to maintain a propercurrent throw relationship so that a uniform thickness and density ismaintained throughout the plated surface of said article.

Another object of this invention is to provide a semiconforming anodeassembly or apparatus for use in an electroplating system of the typewhich is adapted to deposit a significantly heavy functional coating ona non-uniformly curved surface.

Features pursuant to the above objects comprise the use of an anodeassembly having a foraminous of perforate sheet metal titanium wallshaped in a predetermined unique configuration and varied from the shapeof the cathode at selected locations. The wall is retained by end platesformed of the same material, but solid. The cross section of theforaminous wall is defined so that it is semi-conforming with respect tothe shape of the cathode; the anode assembly progressively becomes morespaced from any portion of the cathode article which has a reverselycurved portion, the progression of spacing increasing to a locationintersected by radius of the reversely curved portion passing throughthe mid-point thereof.

Still another object of this invention is to provide a novel and uniquemethod for fabricating an anode assembly which will have a defined crosssectional configuration with a semi-conforming relationship to thecathode, a continuous wall of the assembly being fabricated of perforatesheet metal, such as titanium.

BRIEF SUMMARY OF THE DRAWINGS FIG. 1 is a schematic illustration of anelectroplating apparatus having a stacked series of cathodicallyconsituted articles for plating, and an anode assembly disposed withinthe interior of said series of cathode articles;

FIG. 2 is a plan view of the apparatus of FIG. 1, shown similarly in asomewhat schematic manner;

FIG. 3 is a highly enlarged schematic layout of the cross sectionalconfiguration of the anode assembly and the inner wall of the cathodicarticle to be plated;

FIG. 4 is an exploded view of the basic elements which interfit to formthe cathode assembly according to the method of this invention.

DETAILED DESCRIPTION Turning now to the drawings and particularly FIGS.1 and 2, there is schematically illustrated a preferred mode for ananode assembly and plating system according to this invention. Anelectroplating tank A is provided to contain an electrolyte B, such asan aqueous solution of nickel sulfamate containing inert particles ofsilicon carbide. Typically the bath may contain about 600 grams/liter ofnickel sulfamate, about 120 grams/liter of silicon carbide with a meshsize no greater than 400, about 2.5 grams/liter of a stress relieversuch as saccharin, about 19 grams/liter of nickel chloride, and about 45grams/liter of boric acid (H A cathode assembly C is disposed in theelectrolyte consisting of several cathodic articles 10 each constitutinga cast aluminum rotor housing useful as an element of a rotary internalcombustion engine. The rotor housings are annular and must have a highlywearresistant epitrochoid surface 11 on the interior thereof and againstwhich apex seals or other moving parts of a rotary engine must bear. Thehousings are separated, one from the other, by spacers 12 which may actas shields and prevent plating on the side faces 10a of the rotorhousings. Such spacers can be formed as polypropylene or nylon sheetsand have an inner edge 13 which is recessed from the interior surface 11of each rotor housing. Alternatively, the spacers may be arranged ascathode elements which fit tightly between the housings and which drawcurrent around the edges of the housings to overcome the problem ofexaggerated thickness at such edges; again the spacers would be recessedas illustrated.

At the upper and lower ends of the stack of housings and spacers, thereis employed a rigid annular shield 14 for the top and bottom faces 15and 16 respectively. Each shield should be a plate comprised of aluminumcoated with silicon rubber which stays clean and does not draw plating.Plates are supported by a harness (not shown) which facilitates thelowering and the raising of the entire cathode assembly from theelectrolyte. The harness should similarly be coated so as to have aninert outer surface.

An anode assembly D is employed which is of a semiconforrning typewherein only a portion of the anode 12 is adapted to be proportionedidentical to the cathode surface 1 l to be plated; other portions aredesign ed to progressively deviate from such configuration. The anodeassembly, here, is a basket made from expanded titanium sheet metal (ormay be woven from titanium wire). The walls 19 are foraminous and thebottom and top walls 20 and 21 are each a solid titanium plate. Re-

silient or elastic neoprene bands 22 may be mounted about the anode wall19 to mask off or block the current throw in certain predeterminedelevation zones along the anode assembly, particularly those areas wherethe edges of the cathode article would promote an uneven distribution.The masking also blocks off current throw to the spacing between thehousings. Such masking is unnecessary if cathodic spacers are utilizedas mentioned earlier. Active anode pieces 23, such as nickel, arecollected and stacked in the basket for intimate interengagement witheach other and with the basket.

An anodic film is formed on the titanium basket which affords corrosionresistance and electrical insulation, the basket thereby being renderedanodically inert. The titanium acquires a thin-dense inert oxide filmwhich is chemically resistant to acidic electrolytes and has a highelectrical resistance. The current density, of course, is controlled bythe configuration of the titanium basket even though the nickel anodepieces therein are the active anode metal. Current will pass between thebasket and pieces at a contact point between the anodic film and thenickel pieces; this is so even though the film on the titanium is anelectrical insulation.

To realize the objects of this invention, the anode walls 19 are definedwith a predetermined variation from the nodes 24 of the epitrochoidconfiguration of cathode wall 11. Such nodes or segments have a reversecurvature relative to the uniform curvature of the remaining portions 25and 28 of the epitrochoid; in cross section, the portions 25 maysubstantially be arcs of circles. The cross sectional configuration ofwall 19 has a pair of uniform arcuate segments 26 and 27 which aredirectly proportional and aligned with the segments 25 and 28respectively of wall 11. At stations on wall 19, substantially adjacentthe extremities of the reverse curvature segments 24, varying arcuatesegments 30 and 31, each of which may have a different radius from thatof the uniform segments, are employed. Each of these varying arcuatesegments substantially continue or extend the curvature from each of theuniformly curved segments until they meet at a juncture 32 which is onthe minor axis of the epitrochoid, or in other terms, has a radius 33 ofthe reverse curvature segment 24 passing therethrough and through amidpoint 34 of the reverse curvature. In this manner the total segment30 or 31 is each comprised of two arcs meeting at an abrupt juncture andthereby rendering the combination as varying in curvature.

As shown in FIG. 3, the uniform segments 27 and 26 are formed fromcricles which overlap. It is possible that for some types ofepitrochoids or compound cathode surfaces, the circles should be madetangent. In any event, the deviation (distance 36 distance should beprogressively varied according to the relationship whereby the deviationis inversely proportional to V C.D., where CD. is current density,provided such factors as the conductivity of the solution andtemperature are constant.

METHOD OF MAKING ANODE ASSEMBLY each groove defining a semi-conformingconfiguration to that of the cathode surface. In this case, thesemiconforming configuration comprises two uniform arcs 40 and 41connected by varying segments 42 and 43.

The varying segments are adapted to render a predetermined deviationaway from the cathode surface at these areas to promote uniform platingthickness.

2. Assemble a flexible web of titanium expanded sheet metal (having amesh size no greater than with the longitudinal edges 50 and 51 of theweb in the grooves 46 and 47 respectively. The web is overlapped uponitself at a seam 52 to define a sleeve-like wall with a uniform crosssection reflecting the uniformity and deviations of said grooves.

3. Locate the ends of posts 53 and 54 in mating seats 55 in the endplates to effect a strong stable joint between said plates.

4. Stitch the seam 52 with titanium wire, and fill the assembly with acollection of nickel anode pieces.

5. Provide suitable electrical means for applying a potential to theweb.

We claim as our invention:

1. A method for fabricating an anode assembly for an electroplatingprocess, comprising:

a. preparing a pair of flat impervious titanium end plates with acontinuous groove in each defining an epitrochoid configuration exceptadjacent the minor axis of said configuration whereat said groove isprogressively displaced toward the major axis of the configuration todefine a pair of inwardly directed apices,

b. provide a flat flexible perforate wall of titanium sheet metal andassemble said flexible wall with the edges thereof in the grooves ofsaid end plates, the seam of said wall being secured by titaniumstitching, and

c. locating a pair of spaced posts between said end walls to maintain apredetermined spacing therebetween independent of said flexible wall andfor securing said flexible wall in said grooves so that any section ofsaid flexible wall is maintained as a straight line between grooves inthe opposing end walls.

2. The method as in claim 1, in which the flexible perforate wall isassembled from a wire mesh fabric having a mesh size no greater thanI50.

3. The method as in claim 1, in which pieces of nickel are deposited insaid enclosure defined by said plates and perforate wall, said nickelpieces being in contact with each other, with said perforate wall, andwith at least one of said plates, said nickel pieces providing an activeanode portion and the configuration defined by said plates and perforatewall providing a passive anode portion, said anode portions beingcomplete upon application of current thereto, whereby a thin denseanodic inert oxide film is formed on the surfaces of said plates andperforate end wall thereby affording electrical insulation and assistingin the control of current density.

4. The method as in claim 1, wherein continuous elastic bands of inertmaterial are mounted snugly around the exterior of said flexible wallstationed between said end plates and in said grooves, said bands beinglocated along a predetermined elevation to further assist in controllingcurrent density.

1. A METHOD FOR FABRICATING AN ANODE ASSEMBLY FOR AN ELECTROPLATINGPROCESS, COMPRISING: A. PREPARING A PAIR OF FLAT IMPERVIOUS TITANIUM ENDPLATES WITH A CONTINUOUS GROOVE IN EACH DEFINING AN EPITROCHOIDCONFIGURATION EXCEPT ADJACENT THE MINOR AXIS OF SAID CONFIGURATIONWHEREAT SAID GROOVE IS PROGRESSIVELY DISPLACED TOWARD THE MAJOR AXIS OFTHE CONFIGURATION TO DEFINE A PAIR OF INWARDLY DIRECTED APICES, B.PROVIDE A FLAT FLEXIBLE PERFORATE WALL OF TITANIUM SHEET METAL ANDASSEMBLE SAID FLEXIBLE WALL WITH THE EDGES THEREOF IN THE GROOVES OFSAID END PLATES, THE SEAM OF SAID WALL BEING SECURED BY TITANIUMSTITCHING, AND C. LOCATING A PAIR OF SPACED POSTS BETWEEN SAID END WALLSTO MAINTAIN A PREDETERMINED SPACING THEREBETWEEN INDEPENDENT OF SAIDFLEXIBLE WALL AND FOR SECURING SAID FLEXIBLE
 2. The method as in claim1, in which the flexible perforate wall is assembled from a wire meshfabric having a mesh size no greater than
 150. 3. The method as in claim1, in which pieces of nickel are deposited in said enclosure defined bysaid plates and perforate wall, said nickel pieces being in contact witheach other, with said perforate wall, and with at least one of saidplates, said nickel pieces providing an active anode portion and theconfiguration defined by said plates and perforate wall providing apassive anode portion, said anode portions being complete uponapplication of current thereto, whereby a thin dense anodic inert oxidefilm is formed on the surfaces of said plates and perforate end wallthereby affording electrical insulation and assisting in the control ofcurrent density.
 4. The method as in claim 1, wherein continuous elasticbands of inert material are mounted snugly around the exterior of saidflexible wall stationed between said end plates and in said grooves,said bands being located along a predetermined elevation to furtherassist in controlling current density.